Spectrum analysis is a widely used signal processing technique for gaining information about unknown signals. For programmability and accuracy, digital processing techniques are preferred over analog methods. However for real-time processing of large instantaneous bandwidth signals, analog systems are required for the computationally intensive operations of spectrum analysis. Acousto-optic (AO) based correlators or spectrum correlators are considered to be attractive analog techniques because of their parallel processing capability. Many forms of AO spectrum analyzers have been developed for analog signal processing applications. The different approaches include the AO power spectrum analyzer; the AO in-line time integrating correlator; the interferometric AO spectrum analyzer; and the cascaded AO interferometric architecture.
The major limitation of the power spectrum analyzer is a limited dynamic range of 25-30 dB due to the squaring operation of the instantaneous spectrum of the input waveform at the photodetector array. The in-line time integrating correlator overcomes the dynamic range problems by using a hetrodyne technique at the photodetector array by which provides an output proportional to the magnitude spectrum of the input waveform. The major disadvantage of the in-line correlator architecture is that the spatial frequency of the correlation is fixed with respect to the center frequency of the input signal. The pitch requirement for the photodetector array limits the useful input signal bandwidth of the in-line architecture. The interferometric AO spectrum analyzer uses a spatially and temporally modulated reference beam for generating a fixed spatial frequency at the detector plane while maintaining a high dynamic range for spectrum analysis. The spatial frequency is set by varying the recombination paths between the reference and unknown signal AO inputs in the Mach-Zehnder architecture. The major limitation of this system is that the reference and unknown signals follow widely varying paths in reaching the detector. Thus, the system is extremely sensitive to vibration.
The cascaded interferometric AO architecture overcomes the vibration limitations of the interferometric AO spectrum analyzer by cascading the AO cells such that the reference and signal beam travel along a common path. The interferometric properties of the system are obtained by using two Bragg cells with different acoustic velocities or by interposing a birefringent prism between the two AO cells. The major disadvantage of this approach is the development and use of AO cells with different acoustic velocities or the placement of a prism between the two closely spaced AO cells.